A conventional shell-and-tube heat exchanger is mainly comprised of a shell and heat transfer tubes within it. For the improvement of heat transfer efficiency, baffles are provided in the shell to increase the distance of fluid flow in the shell. In the shell-and-tube heat exchangers of the above structure, however, the clearance between the heat transfer tubes and the through holes of the baffles are relatively large, so the fluid will flow through the clearance directly and the effect of the baffles will decrease. More seriously, impact is likely to occur between the heat transfer tubes and the baffles resulting in breakage of the heat transfer tubes.
In addition, most shell-and-tube heat exchangers use straight heat transfer tubes, with wall thickness generally exceeding 10% of the inside diameter of the tube. Heat exchangers of such a construction have drawbacks of small heat transfer coefficients, being subject to corrosion, scaling readily, occupying large space, and requiring large amount of materials. Although many attempts have been made to improve sell-and-tube hear exchangers, the results are not substantial, with the heat transfer coefficient remaining around 1000 kcal/m.sup.2 .multidot.h.multidot..degree.C. (water-water heat exchange).
U.S. Pat. No. 4,305,460 disclosed a spirally fluted metallic heat transfer tube, wherein the finished tube has the provision of a predetermined number range of multiple start continuous helical flutes formed along its longitudinal length. The helical angle of the flutes induces rotation of the flow within the flutes and of the bulk flow as a result of the curvature of the flutes. The core flow is primarily in solid body rotation, has no strain, and is stable. In the region between the core flow and the flute flow, there is an interchange of angular momentum from the individual flutes to the core flow, resulting in a decrease of the angular momentum in the flutes. This is the case of instability, since the decrease of the peripheral velocity is destabilizing. The instability increases with radially inward heat flow through the wall and decreases with the direction of heat flow outward. Instability enhances the turbulent exchange near the wall, leading to improved heat transfer since most of the resistance to heat flow is in the laminar sublayer.
The heat transfer tube described in the above U.S. Pat. No. 4,305,460, however, lacks flexibility in its longitudinal direction, resulting in being reluctant to undergo elastic deformation in the longitudinal direction, which is not favorable for the heat transfer tube to prevent scaling and to be cleaned of scales. Moreover, in the flow of fluids within the tube there occurs no back-flow essentially and thus no substantial turbulent flow. In addition, because the fluted metallic strips are formed into such heat transfer tubes through opposed contour rollers or by extrusion means, the stress state in the strips is not favorable with many intercrystalline defects readily subject to stress corrosion.